Elizabeth Warren, the Harvard law professor and Massachusetts Democrat, leads incumbent Republican senator Scott Brown by four points, according to a new poll from Suffolk University.

In the poll of 600 likely voters, 48 percent support Warren and 44 percent support Brown. Eight percent remain undecided. The Suffolk poll is the latest in a recent series showing Warren with a small lead over Brown. That's brought the RealClearPolitics poll average to a 3.5-point lead for the Democrat.

Brown, who was first elected to the Senate in a special election in 2010 to complete the late Democrat Ted Kennedy's term, remains popular with Massachusetts voters. According to Suffolk, 60 percent of those polled have a favorable opinion of him, compared to a 52 percent favorability rating for Warren.

Warren has had a few misfires in her campaign, including a protracted public debate over her claims to Cherokee Indian ancestry. Moreover, Brown remains popular and has run a relatively flawless campaign. So how to explain Warren's lead?

First, Massachusetts remains a lopsidedly Democratic state. The Suffolk poll has a partisan breakdown of 36 percent Democrat, 12 percent Republican, and 51 percent "unenrolled/independent" (Massachusetts voters can register as unaffiliated). That 3-to-1 proportion of Democrats to Republicans is in line with the voting population in the Bay State, and it explains a big chunk of Warren's advantage. Brown has to win the entire Republican vote and over two-thirds of the independent vote to stay competitive; he'd have to win nearly all of the independents (and/or a few Democrats) to win. That's what happened in 2010, when Massachusetts voters motivated by the Obamacare debate came out to support Brown, who said he would vote against the bill, over the Democrat, Martha Coakley.

But that was a special election, and this is a presidential year, which explains another part of Warren's lead. The Suffolk poll shows Barack Obama leading Mitt Romney, the state's former governor, 64 percent to 31 percent. The president's popularity in Massachusetts is helping Warren down ballot.

Here's one more interesting tidbit. The poll asked respondents to say the first word or phrase that came to mind upon hearing the name of each Senate candidate. The most common answer for Scott Brown? "Republican," by far. The most common for Elizabeth Warren? "Smart," followed closely by "liar."

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