Tagg Romney (L), the son of defeated US Republican White House candidate Mitt Romney (R), listens as his father speaks during a visit to the home of Beth Sachs in 2008 in Marshall, Michigan.

Washington - Defeated US Republican White House candidate Mitt Romney's son Tagg ruled out running for Senate on Monday after speculation that he might jump in to seek a newly open Massachusetts seat.

“I have been humbled by the outreach I received this weekend encouraging me to become a candidate for the US Senate,” said Tagg Romney, in a statement issued after reports that he was preparing a run.

“I love my home state and admit it would be an honour to represent the citizens of our great Commonwealth. However, I am currently committed to my business and to spending as much time as I can with my wife and children.”

“The timing is not right for me,” he said, a comment that left open the possibility of a future run.

Tagg Romney is Mitt Romney's oldest son and runs a private equity firm.

He had not said he was running in the special June election to fill the seat vacated by newly-confirmed Secretary of State John Kerry, but The Boston Herald had earlier said it “has learned” he was considering a run.

The 42-year-old, a fixture on his father's failed 2012 campaign to oust President Barack Obama, would have been a plausible choice to mount a run this year in Massachusetts, where the elder Romney had been a popular governor.

Republicans are scrambling to find a viable challenger after former senator Scott Brown, who lost his high-profile re-election bid in November to consumer protection advocate Elizabeth Warren, announced he would not run.

William Weld, another popular Republican governor in Massachusetts, has also reportedly opted out of the race, leaving Democrats the strong favourites.

The Herald reported at the weekend that Republican heavyweights were pushing for either Tagg or his mother Ann Romney, who had a star turn on the Republican convention stage during her husband's campaign, to run for the seat.

But a family friend and advisor was quoted as saying that Ann Romney, who has never run for public office, was unlikely to take the plunge.

Democrats are rallying around Congressman Ed Markey, although he could face a primary challenge from fellow Congressman Stephen Lynch. - AFP