The short list of candidates — if there is one — is believed to include Ohio Rep. Rob Portman, former Minnesota Gov. Tim Pawlenty and Wisconsin Rep. Paul Ryan. In a statement issued Friday night, the Romney campaign said the running mate would be revealed at 9 a.m. EDT at the Nauticus Museum. Romney is kicking off a four-day bus tour through swing states.

Speculation has focused in recent days on Ryan, the seven-term congressman. Conservative pundits have been urging Romney to choose Ryan in large part because of his authorship of a House-backed budget plan that seeks to curb overall entitlement spending and changes Medicaid into a voucher-like system to save costs.

Pawlenty was maintaining his Saturday schedule campaigning for Romney in New Hampshire, an official close to Pawlenty’s political team said. The person spoke on the condition of anonymity because the person was not authorized to speak ahead of the formal announcement.

The Wall Street Journal, in an editorial on Thursday, praised Ryan as a strong choice for Romney: “The case for Mr. Ryan is that he best exemplifies the nature and stakes of this election. More than any other politician, the House budget chairman has defined those stakes well as a generational choice about the role of government and whether America will once again become a growth economy or sink into interest-group dominated decline.”

Romney’s choice comes as he tries to repair an image damaged by negative Democratic advertising and shift the trajectory of a campaign that’s seen him lose ground to President Barack Obama.

The vice presidential selection will dominate headlines, and Romney’s team has been relentlessly teasing the announcement for weeks.

Romney’s bus tour was expected to include appearances with Portman, as well as two others talked about as possible contenders for running mate: Virginia Gov. Bob McDonnell and Florida Sen. Marco Rubio.

The tour starts Saturday and will take Romney through four must-win states in as many days: North Carolina, Virginia, Florida and Ohio. All are battlegrounds where Obama won in 2008. While Obama could afford to lose in one or more of them and still reach the 270 electoral votes needed for another term, Romney almost certainly needs all four to beat him.