The Indiana governor is to back Republican presidential candidate Ted Cruz, four days before a crucial primary contest there, US media report.
Campaigning on Friday, Mr Cruz praised Governor Mike Pence, saying he would enthusiastically welcome his support.
Indiana’s primary on Tuesday is the Texas senator’s best chance to stem the momentum of Republican front-runner Donald Trump.
Mr Trump has won a string of states, taking him closer to being the nominee.
But if he does not win 1,237 delegates, there will be a contested Republican convention in July.
This would mean the delegates – local party activists and senior Republican officials – being free to pick an alternative candidate like Mr Cruz.
It has been an eventful week for the Texas senator, taking a beating in five state primaries, picking Carly Fiorina as his running mate and then being called “Lucifer” by ex-House Speaker John Boehner.
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Mr Pence is expected to make his endorsement public later on Friday.
Will this matter? Anthony Zurcher, BBC News, Washington
With Governor Mike Pence endorsing Ted Cruz, the #NeverTrump movement takes a step closer to following a “Wisconsin blueprint” they hope will sink Donald Trump in Indiana and disrupt what has been his increasingly steady march to the Republican presidential nomination.
Mr Cruz’s double-digit victory in Wisconsin earlier this month was fuelled by a confluence of factors – massive outside advertising spending, a determined ground game by the candidate’s campaign team, the support of a popular governor and a local conservative media that was ferociously anti-Trump.
Although the media are much more divided in the Hoosier State -the biggest paper offered a tepid endorsement of Ohio Governor John Kasich, who isn’t even campaigning there – the other pieces are falling into place.
Hundreds of thousands of dollars are being spent by the Cruz campaign and outside groups to take down the Republican front-runner, and Mr Cruz has essentially been criss-crossing Indiana for more than week.
Mr Pence had been under extraordinary pressure to get off the fence in the race – and now he has. If he can he match his endorsement with an energetic final effort to deliver his state to Mr Cruz, the anti-Trump forces may survive to fight another day.