Republican presidential candidate Ted Cruz. (Paul Sancya/AP)

MADISON, Wis. — Ted Cruz sharply warned his fellow Republicans Monday not to try to nominate a candidate for president who is not in the contest right now, saying that such a move would justifiably cause an uproar among party activists.

"This fevered pipe dream of Washington, that at the convention they will parachute in some white knight who will save the Washington establishment — it is nothing less than a pipe dream," Cruz told reporters here before a Fox News Channel town hall. "It ain't gonna happen. If it did, the people would quite rightly revolt."

There has been speculation in Republican circles about the prospect of someone like House Speaker Paul Ryan (R-Wis.) or 2012 nominee Mitt Romney emerging as a consensus choice at the Republican National Convention in Cleveland if  none of the candidates in the race wins a majority of delegates, which is the threshold for clinching the party nod. Both Ryan and Romney have swatted down the possibility that they would swoop in and fill that role.

In his remarks here, Cruz said there was no way his delegates or Donald Trump's delegates would "vote for some uber-Washington lobbyist who hasn't been on the ballot." He did not specify who he was calling an "uber-Washington lobbyist."

The Texas senator was kicking off his final day of campaigning in Wisconsin ahead of Tuesday's primary. Polls show Cruz is favored to defeat mogul Trump and Ohio Gov. John Kasich in Wisconsin. Cruz backers believe a win here would raise raise fresh doubts about Trump's candidacy and give Cruz new momentum in his bid to chase down the front-runner.

The senator framed the battle for the nomination as a one-on-one showdown between him and Trump. But Cruz and his allies have been hitting Kasich hard on the Wisconsin airwaves in recent days, launching a string of attack ads against the Ohio governor.

Cruz expressed confidence that a 2012 Republican National Committee rule stating that candidates be nominated only if they have won delegate majorities in eight or more states would be upheld at this summer's convention. The rule is subject to change. So far, Kasich has won only in his home state of Ohio.