Popular two-term Sen. John ThuneJohn Randolph ThunePoll: 8 in 10 people in key states concerned about driverless cars Hillicon Valley: Mnuchin urges antitrust review of tech | Progressives want to break up Facebook | Classified election security briefing set for Tuesday | Tech CEOs face pressure to appear before Congress Senate GOP urges Trump administration to work closely with Congress on NAFTA MORE (R-S.D.) is set to endorse Mitt Romney Wednesday morning in Iowa, the second major endorsement from the Senate for the former Massachusetts governor this week.

"Mitt Romney has shown throughout his life in the private sector, as leader of the Olympics, as governor and in this campaign that he will not back down from difficult challenges," Thune said in a statement. "Washington could use these common sense principles at such a critical time."

Thune, the chairman of the Senate's Republican Policy Committee, and Romney are set to speak at an event in Iowa at 10 a.m. Wednesday, and then co-host a town hall in the afternoon according to the Sioux City Journal.

“I am honored to have Senator Thune’s support,” Romney said. “On the issues that I have been fighting for in my campaign — creating a better business environment, lessening the regulatory burden, and ending Washington’s spending addiction — Senator Thune has been a leading voice in the Senate. He will be a trusted adviser as I bring this message to voters, work to reverse President Obama’s failed policies, and reform Washington.”

Thune's endorsement comes days after Romney announced the endorsement of two prominent New Hampshire lawmakers: Rep. Charlie Bass and Sen. Kelly AyotteKelly Ann AyotteThe Hill's Morning Report: Koch Network re-evaluating midterm strategy amid frustrations with GOP Audit finds US Defense Department wasted hundreds of millions of taxpayer dollars US sends A-10 squadron to Afghanistan for first time in three years MORE, both Republicans. But in campaigning in the Hawkeye State, Romney is signaling a recommitment to the first-in-the-nation caucus. So far, Romney has been reluctant to devote time and money in the state, skipping prominent conservative events in the state. Romney lost the caucuses in 2008 despite spending heavily there.

While Romney has struggled to create separation from the Republican field, he has held the most consistent support of the GOP candidates, and polls show that the majority of GOP voters expect him to be the party's eventual nominee. Republican lawmakers seem to agree; according to The Hill's list of congressional endorsements, Romney far outpaces his opponents with more than seven times as many backers as Newt Gingrich, who is statistically tied with Romney in most national polls.