Wisconsin Senator Ron Johnson endorses Mitt Romney.

On this morning's "Meet the Press."

... Johnson says Romney is the only candidate with a realistic chance of getting the necessary number of delegates to win the nomination.

Johnson also says the candidates have had “a spirited debate” but he thinks it’s time “to end this” and prepare for the campaign against Obama.

I like the way everything is climaxing in Wisconsin.

ADDED: Meanwhile, Governor Scott Walker is not doing an endorsement.

Walker’s campaign spokeswoman said he’s not picking a favorite because his full attention is on defending himself against a recall election scheduled for June. But some political scientists say that’s only part of the story.

Polls show that the recall will be so close that Walker can’t risk alienating even a few Republican voters by endorsing one presidential hopeful over the others, said Mordecai Lee, a UW-Milwaukee professor of government affairs and former Democratic state lawmaker.

“Walker has no margin of error,” said Lee, pointing to polls that indicate the recall may be as close as the April 2011 state Supreme Court race that was decided by about 7,000 votes out of 1.5 million. “For Walker there is absolutely no self-benefit to endorsing anyone. He’s already a hero to every Republican. Walker hurts himself if he endorses any individual candidate.”