1. Bloomberg’s millions matter more in a House race than a Senate one. A single individual willing to spend heavily on ads in a House district can have much more impact than that same individual spending that same money in a statewide contest. You get more bang for your buck, literally, in a House race than in the Senate.
2. It’s harder to change politicians’ minds on an issue when an election is more than a year off. Bloomberg’s successes in House races were built on timing; his super PAC, which is called Independence USA PAC, went up with a heavy barrage of ads as most voters in each race were paying attention. While Bloomberg is up to something else here — he is trying to influence senators’ votes, not win votes for their opponents — timing still matters a great deal in politics and it’s hard to see how $12 million spent in March of an off-year will have a tremendous persuasive effect on the incumbents.
3. Many of the Senators that Bloomberg is targeting have an easy out if — and, per point number two above, that’s a big “if” — their constituents start to complain about their stance on background checks. Take Arkansas Sen. Mark Pryor, a Democrat up for re-election in 2014 and a target of the Bloomberg ads. If the ads become an issue, Pryor will make sure voters in the Razorback State know the commercials are being funded by the mayor of New York City who, among other things, wants to restrict peoples’ right to smoke and drink big, sugary sodas.