Republicans worried about holding on to their Senate majority are trying to convince three lawmakers to reconsider their decisions and run for the upper chamber.

Reps. Kevin Cramer (R-N.D.) and Ann Wagner (R-Mo.) are being encouraged to launch Senate bids after initially passing on those races, while Sen. Bob Corker (R-Tenn.) is hearing calls to put off his retirement and run for reelection.

The last-minute maneuvering suggests the GOP primary fields may not be settled just eight months before the November midterm elections and highlights an increasing nervousness among Republicans that their Senate majority - once seen as nearly invincible - may be at risk.

"You want to put your best team on the field when you are competing in what could be a potentially challenging election cycle," said GOP strategist Ryan Williams. "I think the fact that there's an effort to coax some people into reconsidering shows a degree of concern over the current crop of candidates."

"If you have your best players in the field, you aren't looking to trade."

Republicans hold just 51 seats in the Senate, which means Democrats could flip control of the chamber with a net gain of two seats.

But the GOP has a huge advantage when it comes to the Senate map. Democrats are defending more than two-dozen seats, including 10 in states that President Trump won in 2016. The GOP is defending just eight seats.

North Dakota

Cramer said he and his wife were seriously reconsidering their decision and would make a final call by this weekend.

"What we didn't calculate was just how overwhelming the encouragement would be," Cramer told The Hill in an interview just outside the Capitol. "The case that many people have made is both just how fragile the United States Senate is and how important it is to the agenda that we ran on and that we supported our president on."

"It's an appeal to patriotism and we feel a patriotic duty to reconsider," he added.

A top Republican strategist confirmed that Cramer has been heavily recruited to reconsider the race by Senate Majority Leader Mitch McConnell (R-Ky.), National Republican Senatorial Committee Chairman Cory Gardner (Colo.) and "the entire crew of Republicans in North Dakota and D.C."

Trump, who has made public overtures for Cramer to run, is also still urging him to rethink the race. The two men have spoken numerous times in recent days, said sources familiar with the calls.

Gary Emineth, a former GOP state party chair, dropped out of the race amid talk of Cramer's likely entry. State Rep. Tom Campbell is the only Republican left in the primary, but he doesn't have the name ID of Cramer, the at-large congressman who has won statewide.

Tennessee

The situation is different in Tennessee, where conservative GOP Rep. Marsha Blackburn is running for Corker's seat and is showing no signs that she'll bow out if the incumbent reconsiders.

Republicans are nervous about holding the seat because Democrats have recruited former Gov. Phil Bredeson for the race.

Blackburn declined to comment Tuesday about whether she would drop out of the race if Corker runs, incorrectly stating that she could not discuss politics while standing on "federal property." But her spokeswoman, Andrea Bozek, dismissed complaints about the Blackburn campaign, calling anyone who doubts the congresswoman's ability to win a "plain sexist pig."

"It's quite literally the old boys network trying to stop her from being the candidate, but she's been doing this for a long time," said a national Republican strategist familiar with the race. "There's no reason to run against her."

Corker announced his retirement amid a public spat with Trump over his mishandling of violent white supremacist protests in Charlottesville, Va., but is being urged by Sen. Lamar Alexander (R-Tenn.) and other colleagues to reconsider. Corker apparently is "listening" to his friends and colleagues but has made no final decision, sources said.

McConnell has told Corker he must make peace with Trump and win the president's backing if he wants to reenter the race, The New York Times reported. Trump has already been in touch with Blackburn, a firm ally of the president who is now the favorite to win the GOP nomination.

Given Trump's silence - he has yet to publicly weigh in on the matter - and the tough words from Blackburn, some doubt Corker will get into the race.

"If [Corker] does run, there's no way he can beat her in a primary. In fact I think she'd crush him in a primary," the national GOP strategist said. "It's not a great way for Corker to end his career. That's why I'm skeptical he would run."

Missouri

Rumors about Wagner giving the Senate race in Missouri a second look came amid a lackluster fourth-quarter fundraising haul for state Attorney General Josh Hawley, as well as backlash over his comment that linked the 1960s sexual revolution to modern human trafficking.

The Republican AG pulled in just shy of $1 million in the final quarter of 2017, the largest amount raised by a GOP challenger running against a Democratic incumbent. Meanwhile, two-term Sen. Claire McCaskill (D-Mo.) raised nearly $3 million during that same quarter, bringing her cash on hand total to a whopping $9 million.

In a brief interview with The Hill on Wednesday, McCaskill cited the sex scandal engulfing the governor's office in calling the Missouri GOP a mess. "I'll take on whoever they nominate," McCaskill said.

Republicans still believe Hawley is a promising candidate, but are closely keeping tabs on his performance to see if he can quickly improve his campaign and fundraising chops.

Hawley's team has touted that he raised more than any other GOP challenger taking on an incumbent and pointed to low favorability numbers for McCaskill. His spokeswoman, Kelli Ford, has pushed back on criticism from McCaskill over his recent comment and highlighted his work combating sex trafficking as Missouri's attorney general.

But many Republicans back in the Show Me State aren't convinced. Wagner, a former Republican National Committee official and ambassador under President George W. Bush, has been fielding phone calls, emails and text messages from everyone from grass-roots activists and donors to major political figures, GOP sources said.

Hawley "needs to start running a campaign. We're a retail-politics state. You've got to be out there," said one Missouri Republican who's been watching the race. McCaskill's "sitting on $9 million; he's sitting on $1 million. And she's working it; she's retail. That's how we do it in Missouri."

For her part, Wagner said she's completely focused on running for a fourth term in the House representing the St. Louis suburbs.

"I've got important work to do in Missouri's 2nd Congressional District. Nothing has changed," Wagner told The Hill in a recent interview. "I am not analyzing anybody else's campaign. It's not my job to speculate or analyze."

Ben Kamisar contributed.