Employer and employee groups alike are attacking a Senate immigration  reform bill that balances tougher border and workplace security with a path to legal status for the nation's estimated 12 million undocumented workers.

Under the bill, which the Senate is expected to take up early next week, undocumented workers would pay a $5,000 fine and return to their home countries before applying for a renewable work visa. As visa-holders, they could then seek permanent U.S. residency, a process that could take up to 13 years to complete. It would also create a guest-worker program for new immigrants.

The initiative is the result of a bipartisan agreement and is supported by the president.

"Politics is the art of the possible and the agreement we just reached is the best possible chance we will have in years to secure our borders and bring million of people out of the shadows and into the sunshine of America," Sen. Ted  Kennedy (D-Mass.), the main Democrat sponsor of the bill, said on Thursday.

Yet, even Kennedy admitted the bill "isn't perfect."

Some Senate Republicans have likened the visa provision for undocumented workers to amnesty for illegal aliens.  

"Our members are concerned about any measure that includes amnesty," said Michael Donohue, a spokesman for the National Federation of Independent Business, a Washington-based small-business lobby.

In a survey of 10,000 of its members last year, 90 percent said illegal immigration is a problem that threatens to inflate taxpayer costs and lower wages, among other issues.

Another key issue for small-business owners is the employment-verification system, according to Donahue. While smaller employers support tougher security measures, he said, they're also worried about being held fully accountable for the legal status of new hires.

The proposed bill would implement a high-tech worker identification program at workplaces nationwide, and includes fines for employers that hire illegal workers. On Wednesday, President Bush said holding employers to account is "an integral part of a comprehensive immigration reform package."

Donahue said small-business owners support the approach, so long as recognizes the differences between small and large employers.

"Getting a $10,000 fine could be devastating to a small business," he said.

At the same time, immigrant and employee rights groups say the measure would lower labor and workplace standards across the board by creating an underclass of exploited temporary workers.

"While today's deal reach on immigration provides an opening for comprehensive legislation on this critical issue, it has a long way to go before it can provide meaningful reform," The United Food and Commercial Workers International Union said in a statement.

The House is expected to take up its own immigration reform bill  in July.