Sen. Marco RubioMarco Antonio RubioTrump replaces McMaster with Bolton as national security adviser Orlando March for Our Lives protesters to march to Rubio's downtown office, Pulse nightclub Lawmakers eye crackdown on China’s Confucius Institutes MORE (R-Fla.) said Wednesday he will seek reelection, a major reversal that greatly helps Republicans’ chances of keeping control of the Senate.

Rubio issued a statement about his change of heart, saying he remains frustrated by the upper chamber’s gridlock and touting his work for constituents. But he noted that his decision predominantly hinged on the Senate’s role in reining in executive power.

“As we begin the next chapter in the history of our nation, there’s another role for the Senate that could end up being its most important in the years to come: The Constitutional power to act as a check and balance on the excesses of a president,” Rubio said.

While hitting presumptive Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonWhite House would like to see Biden ‘in the boxing ring’ in 2020 House Judiciary chair subpoenas DOJ for FBI documents The suit to make Electoral College more ‘fair’ could make it worse MORE, Rubio also took aim at her opponent, Donald TrumpDonald John TrumpPoll: Both parties need to do more on drug prices Senate approves .3 trillion spending bill, sending to Trump White House: Trump will delay steel tariffs for EU, six countries MORE, noting that his former GOP rival is “worrisome.” Rubio noted his continued commitment to “stand up” to the controversial real estate mogul.

“It is no secret that I have significant disagreements with Donald Trump. His positions on many key issues are still unknown. And some of his statements, especially about women and minorities, I find not just offensive but unacceptable,” Rubio said.

“If he is elected, we will need Senators willing to encourage him in the right direction, and if necessary, stand up to him,” he added. “I’ve proven a willingness to do both.”

 A Senate aide told The Hill earlier on Wednesday that Rubio, in an effort to rally conservative support, reached out to Sen. Mike LeeMichael (Mike) Shumway LeeConservatives balk over funding bill ahead of shutdown  Overnight Defense: Senate sides with Trump on military role in Yemen | Dem vets push for new war authorization on Iraq anniversary | General says time isn't 'right' for space corps Senate sides with Trump on providing Saudi military support MORE (R-Utah) to alert him of his intent to run for reelection.

Rubio had insisted since he ended his presidential run that he would retire from the Senate after one term and become a “private citizen.”

But the Florida Republican had been rumored in recent weeks to be reconsidering. 

Top Senate Republicans — led by Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellSenate approves .3 trillion spending bill, sending to Trump GOP senator threatened to hold up bill over provision to honor late political rival: report Paul: Shutting down government not my goal MORE (Ky.) — were urging him to run for a seat that could determine which party controls the chamber.

“He would not only save a terrific senator for the Senate, but help save the majority,” McConnell said of Rubio last month.

Rubio started to publicly re-evaluate his decision following the June 12 mass shooting at a gay nightclub in Orlando.

“I’ll go home later this week, and I’ll have some time with my family, and then if there’s been a change in our status, I’ll be sure to let everyone know,” Rubio said. Friday is the last day he can file to run for the seat.

Following the news Wednesday, Republicans praised Rubio’s decision and quickly rallied behind his candidacy.

Sen. Roger WickerRoger Frederick WickerMiss. governor names state's first female senator to replace retiring Cochran Co-founder of WhatsApp: 'It is time. #deletefacebook' Lawmakers zero in on Zuckerberg MORE (Miss.), chairman of the National Republican Senatorial Committee, said in a statement that Rubio will have the “full support” of the Senate Republicans’ campaign arm.

Sen. John CornynJohn CornynSenate approves .3 trillion spending bill, sending to Trump GOP senator threatened to hold up bill over provision to honor late political rival: report Overnight Health Care: House passes .3T omnibus | Bill boosts funds for NIH, opioid treatment | Senators spar over ObamaCare fix | 'Right to Try' bill heads to the Senate MORE (Texas), the No. 2 Senate Republican, tweeted his support for Rubio, and Sen. Ted CruzRafael (Ted) Edward CruzConservatives balk over funding bill ahead of shutdown  Confirmation fight over Trump pick exposes blurred lines in GOP-LGBT activism GOP pushes to change Senate rules for Trump MORE (Texas) voiced support of his former presidential rival.

Additionally, the conservative Club for Growth PAC and FreedomWorks PAC sent out press releases endorsing 

With Rubio’s announcement, the major Republican candidates seeking to replace him began clearing his path.

Carlos López-Cantera, a close friend for whom Rubio had campaigned, bowed out of the race on Wednesday, saying he’ll continue serving as Florida’s lieutenant governor. López-Cantera reportedly had urged Rubio in a private conversation to run again in the wake of the Orlando shooting.

He asked the remaining GOP candidates to unite behind Rubio.

Rep. Ron DeSantis also dropped out of the race on Wednesday and said he will run for reelection to his House seat. Last week, Rep. David Jolly ended his Senate bid and is instead running for reelection to his redistricted House seat.

The remaining GOP field includes businessmen Carlos Beruff and Todd Wilcox.

Democrats were quick to criticize Rubio’s change of heart.

The Democratic Senatorial Campaign Committee accused him of “cravenly” seeking to capitalize on the Orlando massacre.

The Florida Democratic Party also released a statement making a similar claim, calling Rubio “a shameless opportunist who walked away from his job in the Senate after accomplishing nothing,”

Florida Democratic Party Chairwoman Allison Tant said, “The fact that he sat at the crime scene in Orlando and strategized with the lieutenant governor of Florida about how he could best turn this tragedy to his political advantage disqualifies him from holding public office.”

Rubio’s decision is welcome news to Republicans, who view him as their best shot at keeping the seat in GOP hands.

A Quinnipiac poll released Wednesday morning showed Rubio ahead of both Democratic challengers, 7 points over Rep. Patrick Murphy and 8 points over Rep. Alan 

Murphy, the leading Democratic candidate who has earned support from the party establishment, slammed Rubio on Wednesday over his decision to run.

“It’s clear Sen. Rubio has always put his ambitions above the people he’s supposed to be representing,” Murphy said on a Wednesday afternoon call with reporters. “How can Floridians trust him, someone who continuously breaks his word?”

Murphy said he believes Rubio’s decision didn’t just come out of thin air and said he’s “in the hands” of McConnell and Trump. “I think this has been orchestrated for some time now,” Murphy said. “There’s far too many coincidences that this happened abruptly.”

Grayson campaign manager Mike Ceraso also sent a statement, saying “it’s shameful that Marco is trying to use the Orlando tragedy to further his 2020 presidential ambitions from a Senate seat that he’s barely sat in.”

While Rubio should be in a good position to raise enough funds for a competitive bid, he’ll need to rebuild his stores. He only has $24,000 in the bank and is $1.9 million in debt, according to May fundraising reports.

Shortly after announcing his decision, Rubio’s campaign emailed a fundraising request that outlined his reasons for changing his mind.

Asked on CNN whether he plans to serve out a full six-year term or cut his term short to again seek the White House, Rubio replied: “I’ll tell you this: If my plan was to run for president in 2020, jumping into a race like this with all the political risks associated with it would not be the decision one would make. 

“I’m fully prepared for the U.S. Senate to be the last political job I ever have, and I just want to be the best senator I possibly can be for the people of Florida.” 

Alexander Bolton and Jessie Hellman contributed.

Updated 4:24