CONCORD, N.H. — After a week’s vacation, and facing questions about a relatively subdued campaign for the Republican presidential nomination, Mitt Romney kicked off his busiest week yet on Monday and wasted no time in blaming President Obama for America’s credit rating downgrade.

Mr. Romney accused the White House of inverting President Harry S. Truman’s political motto, “The buck stops here,” into “The buck stops somewhere else,” and called Mr. Obama’s office a “blame presidency.”