Popular two-term Sen. John ThuneJohn Randolph ThuneMcConnell names Senate GOP tax conferees Overnight Health Care: 3.6M signed up for ObamaCare in first month | Ryan pledges 'entitlement reform' next year | Dems push for more money to fight opioids Dems push for more money for opioid fight 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 AyotteExplaining Democratic victories: It’s gun violence, stupid Trump voter fraud panel member fights back against critics Dems plan to make gun control an issue in Nevada 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.