Yesterday, Public Policy Polling reported that Marco Rubio holds a commanding 32-point lead in the Republican primary over Governor Charlie Crist, and today, the news doesn’t get any better.  With the Republican nomination for the open Senate seat all but decided, some have suggested that Crist might run as an independent — although Crist himself has continued to deny any plans to do so.  The latest polling explains why (h/t Randy Scudder on Twitter):

An independent Charlie Crist bid for the US Senate would work to Marco Rubio’s advantage, because Crist would win more Democratic votes than Republican ones. In a hypothetical three way contest Rubio leads with 34% to 27% for Crist and 25% for Kendrick Meek.

Crist gets 32% of the Democratic vote but only 18% of Republicans running as an independent. He also leads among independents with 35% to 24% for Rubio and 22% for Crist.

Crist’s overall approval rating now is a 35/51 spread. He’s most popular with Democrats at a 45% approval rating followed by independents at 29% and Republicans at 28%.

It’s not all bad news, though, and some of the PPP poll supports the argument that Republicans run a risk with Rubio.  In head-to-head polling against the Democratic challenger Kendrick Meek, Crist is significantly ahead, 46/33.  Rubio only has a 44/39 advantage, though, and that’s with a higher level of undecided Democratic voters.  Crist’s popularity with independents is also a potential trouble spot.  However, Crist takes more from Democrats than Republicans in a three-way race, which would ensure Rubio’s election.

PPP surveyed registered voters, however, and not likely voters.  The turnout model matters, especially among independents.  Which unaffiliated voters will be more likely to vote in November — those who wholeheartedly support the Obama agenda, or those angered by it?  While Crist may have an advantage with registered independents, those supporting him will be less motivated than those supporting Rubio.  Similarly, the Democrats turning out for Crist may be less motivated than those turning out for Meek, and much less motivated than Republicans anxious to deliver a referendum on Barack Obama.

Meanwhile, Rubio has his first campaign ad out today, a one-minute spot that shows that he has begun to look past Crist to the general election (via Mary Katharine Ham):

Notice that he doesn’t mention Crist at all in this spot. He’s already running against the Obama agenda. It’s an excellent start to the general-election campaign.