Democratic former Boston mayor Ray Flynn offers a full-throated endorsement of Sen. Scott Brown (R-Mass.) in the newest ad in the closely watched Massachusetts Senate race.

Flynn’s ad came the same day that independent New York Mayor Michael Bloomberg endorsed Brown and announced that he would hold a fundraiser for him.

What’s clear: Brown has crossover support. And some big-name crossover support at that. But just how important — and significant — is it?

Among the Democrats who have expressed support for Brown are:

- Flynn

- Former Worcester (the state’s second-largest city) mayor Konnie Lukes

- Former Obama car czar Steven Rattner, who expressed support for Brown (and distaste for Warren) during an appearance on “Morning Joe.”

- Medford City Councilman Rick Caraviello

- Four former state representatives

- One former state senator

And regular Democratic donors/fundraisers who have crossed over to support Brown include:

- Caritas Cristi CEO Ralph de la Torre, who has raised and contributed lots of money for Senate Democrats, including Brown’s 2010 special election opponent, Martha Coakley

- Real estate investor Jonathan Davis, who contributed to Coakley last time

- Samuel Adams Brewery founder Jim Koch

None of these endorsements and donations, it should be noted, are huge game-changers in and of themselves or demonstrate a widespread Democratic backlash against Warren.

Democrats are quick to note that Flynn has crossed the aisle before — including for Brown. He also backed Brown in the 2010 special election and supported George W. Bush in the 2000 presidential race and Republican Paul Cellucci in the 1998 Massachusetts governor’s race.

Likewise, Bloomberg is something of a political wild card, backing both Republicans and Democrats over the years.

Among the rest of Brown’s crossover supporters are a lot of former elected officials, which are great, but aren’t exactly front-of-mind for a lot of voters. Flynn, for example, hasn’t been in office since 1993.

At the same time, it’s rare that a politician in a marquee race can put together significant crossover support. Politics is a party game, after all, and crossing over comes with consequences for whoever is playing the role of Benedict Arnold (which is why we see it more from former elected officials than current ones).

And Brown has had some success.

Brown needs to show crossover support, though, because he needs Democratic-leaning voters to support him in the bluest state in the country. Flynn and Bloomberg can help in that regard, by essentially leading the way for Democratic voters and Democratic-leaning independents who aren’t used to voting for Republicans.

It’s a work-in-progress for Brown right now.

A recent Suffolk University poll showed him winning 15 percent of the Democratic vote and 60 percent of the independent vote, and he was still in a virtual tie with Warren.

He will need to increase both numbers as the race goes on — especially in a presidential year in which President Obama is likely to carry the state by around 20 points.