Sam Brownback, the conservative Republican who pulled out of the presidential race last month, on Wednesday threw his support behind former rival John McCain.
Brownback, a Catholic, has had strong backing of evangelical Christians, a group McCain has worked to try to win over.
"While I respect all of the Republicans running for president this year, John McCain is the only candidate who can rally the Reagan coalition of conservatives, Independents and conservative Democrats needed to defeat Hillary Clinton or any other Democrat in the general election next year," Brownback said in a statement.
Brownback is expected to join McCain Wednesday on a campaign swing through Iowa and Michigan.
The nod could provide a much-needed boost, particularly in Iowa, for the Arizona senator and one-time presumed GOP front-runner whose bid faltered and who now is looking for a comeback.
It's uncertain how much weight the Brownback's backing will carry; the Kansas senator dropped out of the race last month with little money and little support. While he is a favorite of religious conservatives, he failed to persuade them to embrace him as the GOP's consensus conservative candidate. He spent months emphasizing his rock-solid opposition to abortion, gay marriage and other issues important to the party's right flank, but left the race ranking low in national polls and state surveys.
Still, Brownback's backing could signal to evangelical Christians that they can trust McCain and could help solidify McCain's credentials on social issues. The endorsement could be especially important in Iowa, where McCain trails in polls.
Despite a solidly conservative Senate voting record on social issues, McCain has a rocky history with cultural and religious conservatives who make up a significant part of the Republican base — and have proven to be influential in Iowa's GOP caucuses.
He once likened their leaders to "agents of intolerance," but since has taken steps to heal his relationship with the voting group. Still, some are skeptical that McCain will be a loyal Republican who will champion their issues, partly because while his record is clear cut, he's not a high-profile crusader against abortion rights and gay marriage.
Other candidates in the crowded GOP field had lobbied for Brownback's support over the past few weeks.
Mike Huckabee, the former Arkansas governor and Southern Baptist minister who has made strides in Iowa in recent weeks, was widely considered the other Republican most likely to get Brownback's endorsement. It's unclear whether Fred Thompson, the former Tennessee senator trying to emerge as the conservatives' choice, ever got a look from Brownback.
Brownback did, however, talk with Rudy Giuliani, a backer of abortion rights and gay rights, and emerged from the meeting with kind words about the former New York mayor. Yet, days earlier, Brownback told reporters he saw no way in which the GOP would nominate a "pro-choice" Republican. It's doubtful that Mitt Romney, the former Massachusetts governor who reversed course on abortion, was ever in the running. Brownback spent months this summer bitterly criticizing Romney's shifts on social issues.
The Associated Press contributed to this report.