Punctuating however depends on where it falls in a sentence.
At the beginning or the end, set it off with a comma:
However, American presidential campaigns seem to go on forever.
American presidential campaigns seem to go on forever, however.
If however occurs in the middle of a sentence, use commas around both sides of the word (see cartoon above):
American presidential campaigns, however, seem to go on forever.
If it comes between two complete sentences you have a couple of choices:
Use a period and a capital letter: American presidential campaigns seem to go on forever. However, people are looking for ways to shorten the process.
Use a semicolon: American presidential campaigns seem to go on forever; however, people are looking for ways to shorten the process.
What you can’t do is put commas around both sides of however:
American presidential campaigns seem to go on forever, however, people are looking for ways to shorten the process. <———- This is a no-no.