With a packed audience of many pro-immigration legislation supporters, rows of them "Dreamers" — the young people who were brought to the U.S. as minors and now find themselves in adulthood without legal status — the Senate Judiciary Committee gaveled in for the first of what is expected to be many days of long hearings.

The exchanges were political and pointed, but not without substance. Senators spent most of the day on what is perhaps the toughest element before them: how to determine when the Southwestern border is secured to stop illegal crossings, a key trigger for when those already here can begin the decade-long process of legalizing their status.

"We have come up with a fair bill where nobody gets everything they want," said Sen. Charles E. Schumer (D-N.Y.), a chief architect among the bipartisan group of eight senators who proposed the bill, as the morning session opened.

"We know the status quo is unacceptable. We also know there are many who will want to kill this bill," he said. "Be constructive. We are open to changes. But don’t make an effort to kill a bill that is the best hope for immigration reform in this country."

Republicans, worried that without more specific triggers, there is no guarantee the nation can prevent a new wave of illegal crossings, want to beef up the border security provisions in the bill.

Sen. Charles Grassley of Iowa, the top Republican on the committee, argued that the bill as written is "weak," and would allow the legalization process to begin before illegal crossings are fully blocked.

Without tougher measures, Grassley and others warned, Congress would be repeating the mistakes of the last immigration overhaul, under President Reagan in 1986, as the population of those without legal status has grown.

"No one can dispute that this bill is legalization first, enforcement later," said Grassley, who said he voted for that earlier immigration overhaul, but "we screwed up."

"We need to get it right," he said.

The 844-page bill, which immediately grew by a few dozen pages with the first cleanup amendment supported by the bipartisan group, is complex in both politics and policy.

It would require the federal government to submit a plan to gain 90% control of the border’s most trafficked areas, adding funds for drones, customs officers and local prosecution of illegal entries. New guest worker programs would be established, particularly for low-skilled workers, and employers would be required to verify the legal status of all employees.

In exchange, the estimated 11 million people now in the country, who entered illegally or overstayed visas, would be eligible for provisional status if they begin to pay fees, fines and taxes. After 10 years, they could gain legal residency. After 13 years, they would be eligible for citizenship.

The eight senators in the bipartisan group, four of whom are on the Judiciary Committee, have pledged to fend off changes that would derail the bill. They did so Thursday.

Several of the GOP-backed border control amendments were rejected, with the help of two Republicans, Sens. Lindsey Graham of South Carolina and Jeff Flake of Arizona.

Those votes showed the rift within the GOP, and just how politically exposed the Republicans in the bipartisan group may be with their colleagues.

When Grassley proposed guaranteeing the border is secure for six months before immigrants can begin to apply for legal status, Flake, who just took office this year, argued against making them wait.

"They’re in the shadows and we’ve got to bring them out," said Flake. "We’ve got to know who’s here."

The senators similarly rejected an amendment from Sen. Ted Cruz (R-Texas) that would have tripled border patrol officers and quadrupled the amount of drones and other surveillance technology on the border.

Flake argued the $30 billion to $40 billion cost of those changes would be untenable.

By midday, the bill remained mostly intact.

At one point, Graham was trying to shift the debate away from border control, making the case that the new guest worker and employee verification programs would help stem the flow of illegal entries to the country.

"The people coming over this border live in hell holes," he said. The problem is we cant have everybody who lives in a hell hole coming here. We have to make order out of chaos."

Sen. Jeff Sessions, a Republican from Alabama and a top opponent of the overhaul, stepped in.

"It’s not a hell hole," Sessions said. "There are some great things going on in Mexico. It does have some difficult areas."

Graham revised his comments. "I wasn’t slandering Mexico," he said.

Senators stumbled at times, unsure as they waded through stacks of amendments which way they were voting. When it came time to vote aye or no, senators more than once simply paused and said they did not know.

One key amendment that was put off for another day is what to do with the gay partners and spouses of immigrants.

That issue continues to divide Democrats, who largely support extending immigration rights to gay couples, but know it could derail the bill because of opposition from Republicans.

lisa.mascaro@latimes.com

Twitter: @lisamascaroindc