Russia’s war on Ukraine comes at a moment of political weakness for President Joe Biden. His job approval rating is low, low enough to drag down his party’s chances in the midterms, Americans are angry watching their standard of living diminished by inflation, there is lingering frustration about the president’s handling of COVID and there are widespread doubts whether the 79-year-old Biden — the oldest president in the nation’s history — is physically and mentally up to the job.