During the 2008 presidential campaign, Barack Obama cited a single number again and again in warning that John McCain was not the sort of change agent the country needed: His Senate colleague and presidential opponent had voted with President George W. Bush 95 percent of the time.  

The figure was plucked from the database of CQ Roll Call vote studies, a treasure trove for opposition researchers since the annual assessment of congressional voting patterns began in the early 1950s. And the number — accurate only for the previous year, when McCain tacked right in his pursuit of the Republican nomination — was seen as plenty effective in puncturing the Arizona senator’s reputation as a centrist maverick.  

The selective marshaling of statistics is a necessary skill for politicians as much as it is for policymakers. And the work has been gearing up in recent weeks, as the landscape for the midterm elections becomes more clearly defined and the first congressional primaries (in Texas) loom in only a month.  

A plurality of the attention is already focused, and looks destined to remain, on the quartet of Democratic senators running for re-election in states that Mitt Romney carried in 2012, because how well they fare will go a long way toward determining if Senate control switches to the GOP next year. And so plenty of scrutiny is being given to the glass-half-full, or glass-half-empty, nature of what our 2013 vote studies reveal about how loyal they’re being to both Obama and their party line. As Sen. Mark Pryor runs for a third term in Arkansas — he’s the only incumbent now rated an underdog  by Rothenberg Political Report/Roll Call — he will surely delight in announcing he voted more often against Obama than any other Senate Democrat last year. That will sound much more like a boast than a confession in a place where the president’s approval last year was 35 percent, according to state-by-state approval numbers  released last week by Gallup.  

But Republican Rep. Tom Cotton will just as undoubtedly promote his challenge by describing Pryor’s presidential support score in a way that sounds exactly the opposite, but is just as precise: The sitting senator sided with Obama 90 percent of the time.  

In a similar vein, Sen. Kay Hagan can credibly seek to buttress her very slim advantage in North Carolina — our rating now is Tilts Democratic — by boasting of her political independence this way: On the more than 200 roll calls that were decided along mostly party lines in 2013, she went against the grain more often than all but three of her Democratic colleagues. But it’s also true that whoever wins the crowded GOP contest (the primary’s in May, if a runoff is needed that won’t be until July) will be able to write an accurate television advertisement for the fall chiding Hagan for that same party unity score. That’s because, put another way, it means she stuck with Majority Leader Harry Reid’s team 86 percent of the time.  

The situations are comparable, if a shade less dramatic and more complicated, for the other two on the most-endangered list. First-term Sen. Mark Begich’s race is rated Tilts Democrat, even as Gallup pegs Obama’s approval in Alaska at just 34 percent. For Sen. Mary L. Landrieu, the only true tossup on the Rothenberg/Roll Call map as she tries to secure a fourth term in Louisiana, the president’s poll number is a way-below average 40 percent.  

Both Begich and Landrieu can brag they are on the list of the 10 Senate Democrats who opposed Obama the most last year – even though each of them broke with the president on only 3 percent of the roll calls where he made clear in advance the outcome that he wanted.  

The reason they can be viewed simultaneously as both leading party iconoclasts and 97 percent presidential supporters is because of the unique nature of the Senate Democrats last year: Of the 55 caucus members, 31 either voted Obama’s way every single time or went against his wishes only once. The result was that the group had a higher collective score than any caucus — of either party, in either chamber — has shown for any chief executive since we began calculating presidential support in 1953.  

As they look to market their independent streaks, Begich and Landrieu will be on easier ground if they point to the party unity study. It shows that both broke with their party colleagues on 9 percent of the mostly party-line votes of 2013 – a year in which the average Democratic senator’s score on that test was 6 percent.  

But the more all the nuances of these vote studies get unpacked, the more they can start sounding like a babble of statistical contradictions to all but the most committed analysts. And the typical voter is not among that group. Instead, at a time when the national perception of Congress is that it’s a place paralyzed by hardening polarization and reflexive partisanship, the statistics comporting with that narrative are the likeliest to resonate with the electorate.  

Which means the “red state four” Democratic senators are going to have to labor harder than ever to sell themselves as today’s centrist model — even if they have some numbers to prove it.