Sen. John McCain effectively sealed the Republican presidential nomination yesterday when former governor Mitt Romney abruptly ended his campaign. The Arizona senator immediately turned his attention to repairing relations with disgruntled GOP conservatives and opening the general election campaign with a sharp critique of his Democratic rivals.

McCain signaled a hard-fought fall campaign against either Sen. Hillary Rodham Clinton or Sen. Barack Obama based on conservative principles and built around his national security credentials and reputation as an opponent of wasteful government spending.

He presented his support for President Bush's troop surge strategy in Iraq as a badge of honor and charged that both of his opponents would recklessly adopt a timetable for withdrawing U.S. forces from the conflict without regard for the "profound human calamity" and heightened danger to U.S. security that he said would ensue.

"Often elections in this country are fought within the margins of small differences," he said. "This one will not be. We are arguing about hugely consequential things. Whomever the Democrats nominate, they would govern this country in a way that will, in my opinion, take this country backward. ..."

McCain was especially harsh in his criticism of Clinton and Obama on national security issues, asserting that neither fully recognizes the threat of an Iran with nuclear ambitions and that both will concede to critics of America that this country's own actions in its own defense have helped stir Islamic radicalism.

Arguing that Clinton's and Obama's resolve to combat those threats "will be as flawed as their judgment" about what brought about the threats, he said, "I intend to defeat that threat by staying on offense and by marshaling every relevant agency of our government, and our allies, in the urgent necessity of defending the values, virtues and security of free people against those who despise all that is good about us."

McCain will also run on a biography that has shown character and courage and a willingness to buck convention, and he initially matches up well against both Clinton and Obama, according to the most recent Washington Post-ABC News poll. He holds a statistically insignificant lead over Clinton and trails Obama by a similarly insignificant margin. The poll suggests McCain will have solid support from within his party while also appealing to critically important independent voters.

But he will be running in a year in which energy and enthusiasm lie with the Democrats. His support for Bush's Iraq policies put him at odds with a majority of the country, and his own party has been demoralized since Democrats took over Congress in the 2006 elections. Beyond that is the question of whether a nominee who would be 72 when elected can effectively run as a change-oriented candidate in a year when many voters are clearly seeking a new direction.

The reception McCain received at yesterday's Conservative Political Action Conference, where he was booed loudly when introduced, pointed to the fractured coalition that he must reunite before the fall campaign against Clinton or Obama.

"I am acutely aware that I cannot succeed in that endeavor (of uniting the party), nor can our party prevail over the challenge we will face from either Senator Clinton or Senator Obama, without the support of dedicated conservatives, whose convictions, creativity and energy have been indispensable to the success our party has had over the last quarter century," he said.