PHILADELPHIA — For years President Obama has been a boogeyman to Republicans, uniting them in opposition to a Democrat they feel has taken the country in the wrong direction and failed to lead.

But after a stirring speech at the Democratic National Convention on Wednesday night, many conservatives who cannot bring themselves to support their party’s nominee, Donald J. Trump, credited the president’s speech for being a strong one and expressed regret that Democrats appeared to be appropriating themes that were once the bedrock of their party.

Rich Lowry, editor of the conservative National Review magazine, called the speech “very effective” and noted that the Democrats were using classic Republicanisms.