It doesn't seem like President Obama's endorsement would make much of a difference for these candidates, assuming he only endorsed Democrats, but he's been a fairly popular president among liberals and even some conservatives, so there's a chance that his endorsement could influence some voters. More likely it will raise more awareness of those candidates and perhaps cause more people to vote for that particular office, instead of just not voting for it.
Yes, his endorsement carries influence because he is at an all time high in popularity. Voters are starting to recognize that Obama was an excellent choice for president, and his wife is the most popular political figure in the country. Everything he says will have an impact on this election.
I believe Obama's endorsement will have influence. Even though his presidency is almost over, he has served two terms and has influence with the Democratic party. I believe the decisions Obama makes between now and the end of his term will still have impact, including his endorsement of House candidates.
For many Americans who support and consider themselves to be members of the Democrat Party, Barrack Obama is an enormously popular figure and is often considered to be the best president of the United States in the past several decades. Therefore, his endorsement of candidates will mean an enormous boost to their popularity and chances of being selected.