MILFORD, N.H. — Presidential hopeful Mitt Romney yesterday said he would welcome Republican rival Fred Thompson to the race, but also took some jabs at Mr. Thompson’s long delay in formally announcing his candidacy.

Mr. Thompson, the “Law & Order” television actor and former senator from Tennessee, is expected to officially enter the race this week. Instead of attending a Wednesday night debate in Durham, N.H., Mr. Thompson will be in Los Angeles to appear on “The Tonight Show with Jay Leno.”

“I think it will boost the ratings for Jay Leno’s show, but I’d rather be doing well in New Hampshire,” said Mr. Romney, who is leading in most polls in this early voting state. Mr. Thompson’s candidacy has been a shadow on the GOP contest. He has equivocated about getting into the race, while his campaign organization has been in flux. His entry comes remarkably late in a campaign cycle that began days after the 2006 midterm elections.

“Well, I guess the only comment I’d make to Fred Thompson would be: Why the hurry? Why not take a little longer to think this over?” Mr. Romney jokingly told reporters. “From my standpoint, if he wants to wait until January or February, that would be ideal.”

That’s when the primaries unfold in rapid succession.

Mr. Romney spent yesterday opening his fall campaign, marching in a Labor Day parade in Milford and attending retails stops elsewhere. He said voters would note who is there — and who is not. As for Mr. Thompson’s entry, Mr. Romney quipped: “I think he’ll have some fun. We’re going to presumably have some debates with him. We’ll have had five without him.”