As several polls show a tight race between Norm Coleman and Al Franken heading into the Minnesota U.S. Senate primary, the campaigns – and the media – will be looking for clues as to which candidate is in the stronger position coming out of the primaries heading into the home stretch.

Both candidates are having trouble locking in the independent vote, but Tuesday’s primary will seek to provide a measure of how each has solidified their base.

Coleman is only being challenged by one candidate – the infamous Jack Shepard – who also challenged Coleman back in 2002. Shepherd received 5.6 percent of the vote in that primary, so that will be the baseline measure analysts can look to in judging Coleman’s success on Primary Day. (Assuming, arguendo, that Franken supporters do not cross over and vote for Shepherd to deflate Coleman’s victory margin).

Franken, meanwhile, will face six opponents in the DFL primary, including Priscilla Lord Farris, who has purchased media spots in recent weeks.

Some (right-wing) bloggers have suggested Franken also needs to nab 90 percent of the vote to demonstrate he has the support of the DFL heading into the last two months of the campaign.

On its surface, this sounds reasonable: Five of the six major party U.S. Senate candidates in the 2000, 2002, and 2006 elections received between 89 and 94 percent of the primary vote. However, each of those candidates (Rod Grams, Paul Wellstone, Norm Coleman, Amy Klobuchar, and Mark Kennedy) only faced 1 or 2 primary challengers.

While Franken should receive a larger share of the primary vote than Mark Dayton in 2000 (a plurality victory of 41 percent), that is perhaps a closer parallel as Dayton faced 7 primary challengers. The difference in 2000, however, is that three of those DFL candidates (Mike Ciresi, Jerry Janezich, and Rebecca Yanisch) ran high profile campaigns stretching back several months. Of the six DFL-ers facing off against Franken, only Lord Farris has gained media traction.

It is also difficult to measure the Franken vs. Coleman momentum solely based on voter turnout in their respective primaries. As Smart Politics documented last month, only 17 percent of Minnesota voters cast primary ballots in years in which gubernatorial races are not at stake. Still, voter enthusiasm was evident in 2006 when Klobuchar received 155,486 more votes than did Kennedy on Primary Day (in a gubernatorial election year) – a telling sign of what was to come that November.