They can keep trying to pressure him…..
Check it out:

Officially, Mitt Romney returned to Iowa, the quadrennial presidential proving ground, to give a boost to Joni Ernst. But at a closed-door breakfast fundraiser here Monday, the first question from a donor had nothing to do with Ernst’s Senate campaign.

“When you get elected to the Senate, your job should be to convince Mitt Romney to run for president again,” a donor told Ernst, according to several attendees. The Republican candidate said she would, while Romney laughed.

When Romney and Ernst gathered in a West Des Moines boardroom with about 40 agriculture executives Sunday night, one businessman after another pleaded with Romney to give the White House another shot.