Answer by taking a quote from the following article:

Maurice Robert "Mike" Gravel (; born May 13, 1930) is an American politician who was a Democratic United States Senator from Alaska from 1969 to 1981 and a candidate in the 2008 U.S. presidential election. Born and raised in Springfield, Massachusetts, by French-Canadian immigrant parents, Gravel served in the U.S. Army in West Germany, and he later graduated from the Columbia University School of General Studies. He moved to Alaska in the late 1950s, becoming a real estate developer and entering politics. He served in the Alaska House of Representatives from 1963 to 1966 and also became Speaker of the Alaska House.

In 1980, Gravel was challenged for the Democratic Party's nomination by State Representative Clark Gruening, the grandson of the man Gravel had defeated in a primary 12 years earlier. Several factors made Gravel vulnerable. As an insurgent candidate in 1968, Gravel had never established a firm party base. Not liking to hunt or fish, he was also always culturally suspect in the state. A group of Democrats, including future governor Steve Cowper, led the campaign against Gravel, with Gravel's actions in respect to the 1978 and 1980 Alaskan lands bills a major issue, especially given that the latter's denouement happened but a week before the primary. The sources of Gravel's campaign funds, some of which came from political action committees outside the state, also became an issue in the contest. Another factor may have been Alaska's blanket primary system of the time, which allowed unlimited voting across party lines and from its many independents; Republicans believed Gruening would be an easier candidate to defeat in the general election.  Gruening won the bitterly fought primary, with about 55 percent of the vote to Gravel's 44 percent. Gravel would later concede that by the time of his defeat, he had alienated "almost every constituency in Alaska."  Gruening lost the general election to Republican Frank Murkowski. Gravel was the last Democrat to represent Alaska in Congress for 28 years, until Anchorage Mayor Mark Begich defeated Stevens, by now an aged, iconic figure who had just been convicted of seven felonies for taking unreported gifts, in a very close and protracted election result in mid-November 2008. (The charges against Stevens were subsequently dropped due to prosecutorial misconduct.)

Why were they an issue?