Much has been made of the fact that a substantial portion of North Carolina and Indiana Democratic primary voters who cast ballots on Tuesday for Hillary Clinton told exit pollsters that – if Barack Obama is their party’s nominee this fall – they may vote for Republican John McCain.

Should Obama be concerned? Of course. There is no question that the senator from Illinois must do more to appeal to wavering Democrats, especially white, working-class voters who have heard a lot more about the candidate’s controversial former pastor, the Rev. Jeremiah Wright Jr., than they have about his position on trade policy.

But Obama’s not the only likely party nominee who should be worried about some shakiness at the party base. Despite the fact that all-but-coronated Republican nominee John McCain was running essentially without opposition Tuesday, 27 percent of Republican primary participants in North Carolina cast their votes for a candidate other than McCain. In Indiana, 23 percent of Republican primary voters rejected the senator from Arizona.

Each state saw a portion of the Republican vote go to Texas Congressman Ron Paul, the libertarian, anti-war candidate who has maintained a semi-serious campaign while focusing on getting reelected to the House. But most of the anti-McCain votes went to Republicans who aren’t even running anymore.

In North Carolina, almost 63,000 Republican primary voters – 12 percent of the total – marked their ballots for former Arkansas Governor Mike Huckabee, a Baptist preacher who is far more visceral than the likely nominee on social issues. Another 8 percent went for Paul, while 4 percent – one in every 25 North Carolinians who took GOP ballots – checked “no preference.” In effect, they said that no one at all was better than John McCain.

Almost 20,000 Indiana Republican voters cast their ballots for the living embodiment of no one at all: former Massachusetts Governor Mitt Romney. Twice that number voted for Huckabee, who won 10 percent of the Indiana vote, while Paul took 8 percent.

In both primary states, Republicans in many counties registered even greater opposition to McCain than was suggested in the statewide totals. In North Carolina, for instance, 43 percent of the voters in rural Madison County rejected the presumptive nominee, while a third of the voters in the populous Mecklenburg County cast anti-McCain votes. Most of those votes went to Paul, whose genuinely maverick candidacy has attracted backers who are not at all certain to back McCain in November.

So, while Barack Obama should certainly be concerned about those exit-poll numbers that suggest not all Democrats are enthused about his candidacy, John McCain needs to be at least as worried by actual vote totals that suggest he has yet to “close the deal” with one out of every four Republicans.